Global demand for clean fuels, such as ultra-low-sulfur-diesel (ULSD), has risen quickly because many environmental regulations have been established to substantially lower the sulfur levels of fuels in order to reduce sulfur dioxide (SO2) emissions from use of such fuels.
Hydroprocessing processes have been used to treat hydrocarbon feeds to produce clean fuels. Such processes include hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), which remove sulfur and nitrogen, respectively, from the feeds.
Conventional hydroprocessing processes use trickle bed reactors, in which hydrogen is transferred from a vapor phase through a liquid phase hydrocarbon feed to react with the feed at the surface of a solid catalyst. Thus, three phases (gas, liquid and solid) are present. Trickle bed reactors are expensive to operate and require large quantities of hydrogen, much of which must be recycled through expensive hydrogen compressors. Heat removal from the highly exothermic hydroprocessing processes is inefficient. Significant coke forms on the surfaces of catalysts in trickle bed reactors, causing catalyst deactivation.
Ackerson, in U.S. Pat. No. 6,123,835, discloses a two-phase hydroprocessing system which eliminates the need to circulate hydrogen through the catalyst. In the two-phase hydroprocessing system, a solvent or a recycled portion of hydroprocessed liquid effluent acts as diluent and is mixed with a hydrocarbon feed. Hydrogen is dissolved in the feed/diluent mixture to provide hydrogen in the liquid phase. All of the hydrogen required in the hydroprocessing reaction is available in solution.
Two-phase hydroprocessing systems contain a single liquid recycle stream to increase dissolved hydrogen availability throughout a reactor. The recycle stream eliminates hydrogen gas recirculation through the catalyst and provides a heat sink for a uniform temperature distribution. However recycle has disadvantages. Recycle introduces back-mixing to the system, which reduces conversion, e.g., sulfur removal efficiency. Back-mixing reduces catalyst efficiency because reaction products, such as H2S and NH3, which are present in the recycle stream take up the catalyst active sites. This causes difficulty in competing with conventional trickle bed reactors, which do not have liquid recycle, in kinetically limited regions, i.e., reducing sulfur below 10 ppm for ULSD. By “kinetically limited region”, it is meant herein where organic sulfur concentration is very low (such as around 10-50 ppm). The reaction rate of organic sulfur conversion is reduced, kinetically limited, at such low sulfur concentrations in the presence of recycle, which includes reaction products. Use of a recycle stream introduces reacted sulfur (as H2S) back into the reactor (back-mixing) which reduces rate of sulfur conversion. Thus, sulfur removal efficiency is reduced.
Therefore, it is desirable to have a process for hydroprocessing hydrocarbon feeds in a smaller and simpler system with increased sulfur and nitrogen conversions.